Atmos. Chem. Phys., 10, 8855–8872, 2010 www.atmos-chem-phys.net/10/8855/2010/ Atmospheric doi:10.5194/acp-10-8855-2010 Chemistry © Author(s) 2010. CC Attribution 3.0 License. and Physics The invigoration of deep convective clouds over the Atlantic: aerosol effect, meteorology or retrieval artifact? I. Koren1, G. Feingold2, and L. A. Remer3 1Department of Environmental Science and Energy Research, Weizmann Institute of Science, Rehovot, Israel 2NOAA Earth System Research Laboratory, Boulder CO, USA 3Laboratory for Atmospheres, NASA Goddard Space Flight Center, Greenbelt MD, USA Received: 11 January 2010 – Published in Atmos. Chem. Phys. Discuss.: 10 February 2010 Revised: 1 September 2010 – Accepted: 2 September 2010 – Published: 20 September 2010 Abstract. Associations between cloud properties and those that correlate with observed cloud fields. The result aerosol loading are frequently observed in products derived is a near-orthogonal influence of aerosol and meteorological from satellite measurements. These observed trends between fields on cloud top height and cloud fraction. The results clouds and aerosol optical depth suggest aerosol modifica- strengthen the case that the aerosol does play a role in invig- tion of cloud dynamics, yet there are uncertainties involved in orating convective clouds. satellite retrievals that have the potential to lead to incorrect conclusions. Two of the most challenging problems are ad- dressed here: the potential for retrieved aerosol optical depth to be cloud-contaminated, and as a result, artificially corre- 1 Introduction lated with cloud parameters; and the potential for correla- Aerosol effects on clouds are recognized as contributing sub- tions between aerosol and cloud parameters to be erroneously stantially to anthropogenic effects on climate and the wa- considered to be causal. Here these issues are tackled directly ter cycle. Understanding the different cloud feedbacks initi- by studying the effects of the aerosol on convective clouds in ated by changes in aerosol properties poses one of the great- the tropical Atlantic Ocean using satellite remote sensing, a est challenges in climate, cloud and precipitation physics chemical transport model, and a reanalysis of meteorological and radiative transfer (Ramanathan et al., 2001, Kaufman fields. Results show that there is a robust positive correlation et al., 2002). The strong sensitivity of the climate system between cloud fraction or cloud top height and the aerosol to clouds, and the steadily increasing pressure on water re- optical depth, regardless of whether a stringent filtering of sources, makes this a problem of major importance (IPCC, aerosol measurements in the vicinity of clouds is applied, 2007). or not. These same positive correlations emerge when re- placing the observed aerosol field with that derived from a But why are aerosol-cloud interactions so difficult to quan- chemical transport model. Model-reanalysis data is used to tify? Some of the important aspects of this problem are enu- address the causality question by providing meteorological merated below: context for the satellite observations. A correlation exercise Complexity: The inherent complexity of clouds is such between the full suite of meteorological fields derived from that the system is not amenable to analytical solution, nor model reanalysis and satellite-derived cloud fields shows that to observation or model simulation at the full range of tem- observed cloud top height and cloud fraction correlate best poral and spatial scales. The sensitivity to initial and bound- with model pressure updraft velocity and relative humidity. ary conditions in the form of thermodynamic, radiative, and Observed aerosol optical depth does correlate with meteo- aerosol properties is inherently non-linear, so that small rological parameters but usually different parameters from changes in the initial conditions can propagate to large ones in the size, shape, microphysical properties and evolution of the cloud. It is not always clear which of the non-linear Correspondence to: I. Koren feedbacks will be ignited by changes in the initial or bound- ([email protected]) ary conditions of the system. The aerosol can modify cloud Published by Copernicus Publications on behalf of the European Geosciences Union. 8856 I. Koren et al.: The invigoration of deep convective clouds over the Atlantic radiative properties (Twomey, 1977) but also the ability of the clouds with weak optical signature) decreases as a function cloud to precipitate (Albrecht, 1989; Rosenfeld et al. 1999; of the distance from detectable clouds (Koren et al., 2008b, Andreae et al., 2004) and the pathways via which precipi- 2009). Moreover, aerosol particles may change their (true tation develops (e.g., Khain et al., 2001; Lee et al., 2008). and apparent) optical properties near clouds as the humid- Modification of precipitation influences the dynamics of the ity increases (Charlson et al., 2007, Twohy et al., 2009) and environment by changing the vertical distribution of latent due to enhancement in the mean photon flux as a result of heat. It has been suggested that clouds growing in polluted the clouds serving as secondary photon sources illuminating environments are characterized by more vigorous convection the cloud field from their edges (Marshak et al., 2006; Wen owing to a combination of suppression of early rainout of et al., 2007). All of the above effects: undetectable clouds, the cloud and its attendant stabilization, together with sup- aerosol humidification and the 3-D cloud effects, yield an ap- pression of freezing, and the eventual release of latent heat parent larger Aerosol Optical Depth (AOD) measured from at higher altitudes (Koren et al., 2005; Khain et al., 2005). space, and interfere with properly characterizing the aerosol Aerosol perturbations to deep convective systems may in- measured from ground-based remote sensing or even in situ fluence secondary convection (Seifert et al., 2006; van den measurements. Heever and Cotton, 2007; Lee et al., 2008) and other dynami- Causality: Even assuming that clouds and aerosol can be cal responses that magnify the initial microphysical perturba- measured correctly, the last and the ultimate problem is the tion. Conversely, the feedbacks sometimes exhibit multiple strong coupling of clouds and aerosols to meteorology (en- microphysical, dynamical and radiative effects that counter vironmental properties). Are the observed aerosol-cloud re- one another (Kaufman and Koren, 2006; Jiang and Feingold lationships a result of the aerosol effect on clouds or does 2006; Koren et al 2008) and yield a relatively small overall meteorology drive the changes in both aerosol and clouds effect. It has been suggested recently that aerosol-cloud in- properties? Meteorological conditions control most of the teractions occur within a buffered system so that the response cloud properties. Variables such as temperature, humidity, of the system to the aerosol is much smaller than might have surface fluxes and winds largely determine the depth of con- been expected had internal interactions not been accounted vection and the size of clouds. The major challenge is to find for (Stevens and Feingold, 2009). The challenge is to identify the cloud response to perturbations in the aerosol properties geographical regions or distinct regimes where the aerosol buried beneath the significant natural variability due to mete- effect is likely to be largest. orology. Numerical models that simulate the same scenario Measurement Uncertainties: Clouds are extremely in- and change only the aerosol properties are often used to es- homogeneous and form complex, three-dimensional (3-D) tablish causality but it is difficult to generalize the results for structures, making them difficult to characterize from in situ different regimes. It is not always clear if the observed rela- measurements and causing remote sensing retrievals to be tionships are applicable to a wide variety of meteorological a true challenge (Platnick et al., 2003). The aerosol may be regimes, requiring great effort to decouple basic meteorolog- more homogeneous than clouds, but its measureable signal is ical properties (humidity and temperature) from aerosol ef- weak relative to the background and instrument noise (Tanre´ fects (Teller and Levin, 2008; Altaratz et al., 2008). et al., 1996, 1997; Kahn et al., 2005). The challenge grows An extreme test: Detecting the interaction between aerosol even larger when measuring aerosol properties in the vicin- and convective clouds is an extreme test of all of the above ity of clouds. When attempting to study cloud-aerosol inter- challenges. Convective clouds exhibit the highest inhomo- actions from observations, we often ask for the impossible: geneity and are extremely sensitive to changes in the environ- on the one hand we strive to measure the aerosol as close mental conditions (e.g. atmospheric instability). Due to their as possible to clouds in order to reflect the relevant aerosol high variability, in a given convective cloud field one can ex- properties that interact with the cloud. On the other hand, we pect to find clouds at various stages of their lifecycle, which ask for very accurate measurements of aerosol loading and makes measuring/retrieving cloud properties difficult and in- properties, which are very difficult to achieve in the vicinity troduces artifacts in aerosol properties measured/retrieved in of clouds, especially by satellite remote sensing. the vicinity of detectable clouds.